1. Field of the Invention
The present invention relates to the separation of hydrocarbon impurities from lower alkylene oxides such as propylene oxide by extractive distillation with a glycol extractive solvent.
2. Description of the Prior Art
Monoepoxides such as propylene oxide are highly important chemicals useful in a great number of applications. An important commercial technology for producing the monoepoxides is via the catalytic reaction between the corresponding olefin and an organic hydroperoxide. See, for example, U.S. Pat. No. 3,351,635.
In carrying out this reaction the organic hydroperoxide is reduced to the corresponding alcohol. Also produced, however, are small amounts of other oxygen-containing compounds such as methanol, acetone, acetaldehyde and the like, as well as hydrocarbons which are difficult to separate. In general, the alcohol resulting from the reduction of the hydroperoxide can be separated from the epoxide product by ordinary distillation methods, particularly since the organic hydroperoxide employed can be selected to permit this separation. The small amounts of hydrocarbons and other oxygenated compounds, however, remain as impurities in the olefin oxide product. For certain of the epoxides, it is extremely important that these impurities be reduced to a very low ppm level, e.g., below about 200 ppm and especially below 50 ppm.
Hydrocarbon impurities associated with the lower alkylene oxides are paraffinic and olefinic hydrocarbons having 4 to 8 carbon atoms and in the case of propylene oxide are believed to be propylene derivatives having from 4 to 7 carbon atoms per molecule, primarily derivatives having 6 carbon atoms per molecule. The C.sub.6 compounds include primarily methyl pentenes and methyl pentanes. These materials have boiling points sufficiently close to that of propylene oxide (about 35.degree. C. at 760 mm/Hg) so that they are not effectively separated from propylene oxide by direct fractionation. In some cases an azeotrope is formed, making separation even more difficult.
U.S. Pat. No. 3,843,488 describes the separation of contaminating hydrocarbons from propylene oxide by distillation in the presence of a C.sub.8 to C.sub.20 alkane, alkene or naphthene. U.S. Pat. No. 3,909,366 describes the separation of contaminating hydrocarbons from propylene oxide by distillation in the presence of aromatic hydrocarbons having 6 to 12 carbon atoms. U.S. Pat. No. 3,464,897 shows propylene oxide purification by extractive distillation with aliphatic or cyclic paraffins having 8 to 12 carbon atoms.
It has previously been proposed to separate oxygen-containing impurities from the propylene oxide by extractive distillation using lower glycols such as ethylene glycol and propylene glycol. See U.S. Pat. No. 3,578,568 which describes this procedure and which teaches use of solvent in amount to comprise 15 to 50% of the vapor space in the distillation zone. U.S. Pat. No. 5,000,825 describes a similar separation but using much less solvent whereby propylene oxide losses are reduced.